The invention relates to a process for making liquid melamine urea formaldehyde (xe2x80x9cMUFxe2x80x9d) resins, liquid suspension hardeners, and adhesive compositions containing these materials. In particular, the invention relates to a liquid MUF resin that is stable for a significant period of time.
MUF resins are widely used in the woodworking industry for gluing wood or as binders for the production of wood-based materials, including particle board, plywood, various fiber boards, and the like. These resins are the products of reactions of urea and melamine with formaldehyde. The resin is then set or cured with a hardener, such as ammonium salts or acids.
The resins, however, can often quickly solidify once the reaction between the melamine, urea, and formaldehyde is complete, sometimes in a matter of hours or minutes. This solidifying before the hardener is added, is problematic in that the resin cannot be stored for later use.
U.S. Pat. No. 4,536,245 to Shiau et al. discloses a MUF resin with a formaldehyde to urea equivalent molar ratio of 0.7:1 to 1.3:1. Urea equivalent is calculated by determining the moles of urea used and adding to that value 1.5 times the moles of melamine used. Thus, the urea equivalent molar ratio will be smaller than a straight formaldehyde to urea and melamine molar ratio.
U.S. Pat. No. 4,603,191 to Kong discloses a process for preparing a urea-formaldehyde resin having a very low mole ratio of formaldehyde to urea. It teaches a formaldehyde to urea molar ratio of 1:1 to 1.3:1.
U.S. Pat. No. 4,997,905 to Druet et al. teaches a process for producing a melamine urea-formaldehyde resin in three stages wherein the melamine is added at the first stage, second stage, or both. The final ratio of formaldehyde to amino groups ratio is 0.3:1 to 0.5:1.
U.S. Pat. No. 5,008,365 to Druet et al. teaches a process for producing a urea-formaldehyde resin with no more than 10% melamine. The final ratio of formaldehyde to amino groups is 0.5:1 to 0.575:1.
U.S. Pat. No. 5,162,462 to Barthomieux et al. teaches a process for producing aminoplast resins with 2-ureido-4,6-diaminotriazine-1,3,5 and optionally added melamine. The result is a resin with very low formaldehyde emission rates.
U.S. Pat. No. 5,681,917 to Breyer et al. discloses a method for preparing an MUF resin with a low formaldehyde content. The resin has a formaldehyde to urea and melamine molar ratio of 0.5:1 to 1.1:1.
E.P. Application No. 0501174 discloses a hardening composition for a urea-formaldehyde adhesive that includes an aqueous solution of an ammonium salt as a polymerization catalyst and a polyvinyl acetate resin in an emulsion.
PCT Application No. WO 99/48991 discloses a hardener for use in urea-formaldehyde and urea-melamine-formaldehyde based adhesives that includes a polyvinyl acetate emulsion together with a metal chloride and an ammonium salt.
A urea formaldehyde resin with a high melamine content is described by in xe2x80x9cMelamine Modified Urea-formaldehyde Resin For Bonding Flakeboards,xe2x80x9d Proceedings of a Symposium on Wood Adhesives Before the Forest Products Research Society, pp. 155-59, Madison, Wis., 1990. The resins contained 9.5 to 34.4 percent melamine by weight. These resins, however, had a shelf life of less than 1 day.
The focus of the art up to this point has been to produce a resin that has very low formaldehyde emission rates. These resins either do not have good wet bonding strength or have insufficient stability to be stored for extended periods of time. Thus, there is a need for liquid resins that are stable for extended periods of time, can be stored for later use, and are resistant to moisture in wood bonding applications. Resins that contain a higher level of formaldehyde and melamine are generally less stable than those that contain less formaldehyde and melamine. There is also a need in the art for a liquid resin and liquid hardener that can be used in both thermal and radio frequency curing conditions.
The invention relates to a method of preparing a stable melamine urea formaldehyde resin, including the steps of: providing a first aqueous solution of formaldehyde, heating the solution, adding in a single step about 0.35 to 0.65 molar equivalents of urea based on the amount of formaldehyde provided in the first aqueous solution to form a UF mixture, adding acid to the UF mixture to adjust the pH to a value of about 4 to 7 to provide an acidified UF mixture, monitoring the viscosity of the acidified UF mixture until it reaches a value of about 150 to 1000 cps, adjusting the pH of the acidified mixture to a value of about 8 to 10 to provide a basic UF mixture, adding a second aqueous solution including about 0.24 to 1.27 molar equivalents of formaldehyde based on the amount of formaldehyde provided in the first aqueous solution to the basic UF mixture, adjusting the pH of the basic UF mixture to a value of about 8 to 10, adding in a single step about 0.15 to 0.55 molar equivalent of melamine based on the amount of formaldehyde provided in the first aqueous solution to the basic UF mixture to form an MUF mixture, monitoring the viscosity of the MUF mixture until it reaches a value of about 150 to 1000 cps, and adjusting the pH to a value of about 9 to 10 to provide the melamine urea formaldehyde resin having a viscosity that is less than about 1500 cps for about 14 days.
In one embodiment, the preparation further includes adding up to about 0.13 molar equivalents of methanol based on the amount of formaldehyde provided in the first aqueous solution to the MUF mixture. In a preferred embodiment, the added acid is formic acid. The pH may be adjusted with sodium hydroxide. In a preferred embodiment, the viscosity of the acidified UF mixture is monitored until the viscosity reaches a value of about 150 to 400 cps. The aqueous mixture of formaldehyde is preferably formalin.
In another embodiment, the method includes preparing a melamine urea formaldehyde resin and adding a hardener to the resin to form an adhesive. In one embodiment, the hardener or adhesive is combined with a filler selected from the group of wood flour, pecan shells, organically modified cellulose, various starches, insoluble fibrous carbohydrate fillers, tricalcium phosphate, clay, barium sulfate, and hydrated aluminum magnesium silicates.
The invention further relates to a MUF resin having a formaldehyde to melamine and urea molar ratio of about 1.5:1 to 2.5:1, a melamine to urea molar ratio of about 0.25:1 to 1.5:1, and a viscosity of less than about 1500 cps for about 14 days. In a preferred embodiment, the formaldehyde to melamine and urea molar ratio is about 1.75:1 to 2.3:1, the melamine to urea molar ratio is about 0.5:1 to 0.75:1, the viscosity is about 200 cps to 400 cps, and the viscosity of the resin remains at a value of less than about 1500 cps for at least about 3 weeks.
The invention also relates to a liquid hardener including a catalyst, a filler, a thickener, and water, wherein the filler remains suspended in the hardener for at least about 14 days. In a preferred embodiment, the hardener includes about 6% to 30% of the catalyst, about 7% to 50% of the filler, and about 0.8% to 10% of the thickener. The catalyst is selected from the group consisting of a Lewis acid, a Bronsted acid, an ammonium salt, a substituted ammonium salt, or a combination thereof. Preferably, the catalyst includes AlCl3, NH4Cl, or a combination thereof; the filler includes pecan shells, wood flour, or a combination thereof; and the thickener includes polyurethane, polyvinyl alcohol, fumed silica, or a bentone clay. In another embodiment, the liquid hardener further includes an anti-foaming agent.
The invention also relates to an adhesive composition including a hardener and an MUF resin, the resin having a formaldehyde to melamine and urea molar ratio of about 1.5:1 to 2.5:1, a melamine to urea molar ratio of about 0.24:1 to 1.27:1, wherein the viscosity of the adhesive is less than about 1500 cps for at least about 14 days. In one embodiment, the composition includes about 5 to 35 parts per hundred of the hardener. In a preferred embodiment, the composition has a conductivity of about 1 to 5 mS/cm.
The invention relates to a liquid MUF resin that is stable for extended periods of time and to a process for making the resin. The resin is stable for at least 14 days, preferably at least 3 weeks, and more preferably at least 4 weeks. The resin contains a relatively higher amount of formaldehyde and melamine than is found in prior art resins. The higher level of formaldehyde unexpectedly gives the MUF resin of the invention desired physical properties not present in the prior art resins. These properties include a greater bonding strength and shorter reaction time to prepare the resin. Also, for wood bonding applications, a higher formaldehyde to melamine and urea ratio is desirable because a lower ratio increases the time required to cure the composition once the hardener is added. The final molar ratio of formaldehyde to melamine and urea in the MUF resin of the invention is typically about 2.0 to 1. This MUF resin is prepared according to the following steps:
(a) A first aqueous solution of formaldehyde, at a pH of about 7 to 10, preferably at a pH from about 8.0 to 8.5, is contacted with urea. The amount of formaldehyde is about 60% of the total formaldehyde to be used. Urea is added in an amount of about 0.35 to 0.65 molar equivalents relative to the amount of formaldehyde in the first aqueous solution of formaldehyde. Preferably, the aqueous formaldehyde solution is heated to about 40xc2x0 C. to 60xc2x0 C., and more preferably to about 45xc2x0 C. to 55xc2x0 C., before the urea is added. Once the urea is added, the mixture is heated to about 80xc2x0 C. to 100xc2x0 C., preferably about 85xc2x0 C. to 95xc2x0 C. The pH of the formaldehyde solution can be adjusted with caustic, such as an aqueous sodium hydroxide solution containing up to 50% sodium hydroxide;
(b) A sufficient amount of acid is added to the mixture to cause the urea and formaldehyde to polymerize. Typically enough acid is added to adjust the pH to a value of about 4 to 7, preferably to a value of about 4.9 to 5.1;
(c) The viscosity of the mixture is monitored using a Brookfield cone plate viscometer at 25xc2x0 C. using spindle #42 and shear rate of 10 rpm for a 1 ml sample; when the viscosity reaches about 150 to 1000 cps, preferably about 200 to 800 cps, and more preferably about 250 to 450 cps, the mixture is cooled to about 50xc2x0 C. to 70xc2x0 C., preferably about 55xc2x0 C. to 65xc2x0 C., and the pH is adjusted to a value of about 8 to 10, preferably about 8.4 to 8.8. The pH can be adjusted with a caustic, such as an aqueous sodium hydroxide solution containing up to 50% sodium hydroxide;
(d) Additional aqueous formaldehyde, about 0.24 to 1.27 molar equivalents, preferably about 0.44 to 0.84 molar equivalents, relative to the amount of formaldehyde in the first aqueous solution of formaldehyde is added and the pH of the mixture is readjusted to a value of about 8 to 10, preferably about 8.5 to 8.8;
(e) Melamine is added in an amount of about 0.15 to 0.55 molar equivalents, preferably about 0.22 to 0.42 equivalents, relative to the amount of formaldehyde in the first aqueous solution of formaldehyde, followed by a sufficient amount of methanol, typically up to 0.13 molar equivalents relative to the amount of formaldehyde in the first aqueous solution of formaldehyde, to prevent polymerization of methylol groups that are formed from the reaction of formaldehyde with the melamine or urea, and the pH is then increased to a value sufficient to cause the melamine to react with the formaldehyde. Typically the pH is increased to about 9 to 11, preferably to about 9.6 to 10.5. The reaction is typically heated to about 70xc2x0 C. to 90xc2x0 C., preferably about 75xc2x0 C. to 85xc2x0 C.;
(f) The viscosity of the mixture is monitored until it reaches a value of about 150 to 1000 cps, preferably about 200 to 400 cps, and the reaction is then cooled to about 15xc2x0 C. to 35xc2x0 C., preferably about 20xc2x0 C. to 30xc2x0 C., and the pH is adjusted to a value of about 9 to 10, preferably about 9.2 to 9.5, to finish the reaction.
The melamine and urea are each added in single steps, making the process simpler than the processes used in the prior art. The pH can be adjusted using any suitable caustic or acid. Preferably, the caustic is an aqueous sodium hydroxide solution containing up to 25% sodium hydroxide, or tri-ethanol amine. Preferably, the acid is 10% formic, 5% nitric, or 5% sulfuric acid. The amount of time required for each step varies, depending on the temperature and pH at which the step is carried out. One of ordinary skill in the art can readily determine the reaction time without undue experimentation. Preferably, the aqueous mixture of formaldehyde is a solution of 44% formaldehyde (formalin). Other formaldehyde solutions may also be used, such as a 50% or 37% aqueous solution, or even a urea-formaldehyde concentrate. When the formaldehyde content is more than 40%, the solution may require heating in order to maintain a solution. The amount of each reagent can be varied depending on the amount of MUF resin to be produced, but the molar ratio of the reagents should remain the same.
The formaldehyde reacts with both the urea and the melamine to form a methylol group, xe2x80x94CH2OH. The addition of the methanol esterifies the methylol group to provide a stable resin. Preferably, less than about 2% of the methylol groups are esterified. Alternatively, the urea and formaldehyde can be reacted separately and the melamine and formaldehyde reacted separately and the resulting mixtures then combined together.
The invention also relates to the resulting MUF resin. The MUF resin of the invention has a viscosity of about 150 cps to 1000 cps, preferably from about 200 to 400 cps and is stable for at least about 14 days, preferably at least about 3 weeks, more preferably at least about 4 weeks. The term xe2x80x9cstablexe2x80x9d or xe2x80x9cstabilityxe2x80x9d as used herein means that the length of time at a temperature of about 25xc2x0 C. before the viscosity of the resin reaches 1500 cps, preferably before it reaches 1000 cps. The MUF resin is still useful, however, at viscosities up to about 2000 cps. The resin typically has a solid content of about 40% to 80%, preferably about 50% to 70%.
The MUF resins typically have a molar ratio of melamine to urea of about 0.25:1 to 1.5:1, preferably about 0.45:1 to 0.8:1, and more preferably about 0.5:1 to 0.75:1. In one example, the melamine to urea ratio was about 0.64 to 1. The resin typically has a formaldehyde to melamine and urea ratio of about 1.5:1 to 2.5:1, preferably about 1.75:1 to 2.3:1. In one example, the formaldehyde to melamine and urea ratio was about 2.0 to 1. The resin is advantageously more water resistant than conventional resins, due to its high melamine content. The resins also have a higher formaldehyde content than the prior art which is believed to make the resins stronger and more reactive.
The invention further relates to a hardener that can be mixed with the resin to form a adhesive composition for use in wood bonding. The hardener includes a catalyst, a thickener, a filler, and water. The hardener includes about 6% to 30%, preferably about 7% to 17% of a catalyst; about 7% to 50%, preferably about 8.5% to 45% of a filler; and about 0.8% to 10% of a thickener; with the remainder of the composition made up of water.
The catalyst is used to catalyze the curing of the adhesive during wood bonding applications. Without wishing to be bound by theory, it is believed that this is accomplished by cross-linking of the resin during curing. The catalyst may be a Lewis acid or a Bronsted acid and may be an organic or inorganic acid. The catalyst may also be an ammonium salt, such as, for example, NH4Cl, or a substituted ammonium salt. As used herein, the term xe2x80x9csubstituted ammonium saltxe2x80x9d refers to an acid addition salt of any aliphatic or aromatic amine.
Examples of Lewis acid catalysts include, but are not limited to, AlCl3, AlBr3, Al2(SO4)3, MgCl2, MgBr2, along with other Lewis acids, in particular Lewis acids of Ca, Sr, Ti, Fe, Zn, Sn, Sb, Zr, Hg, TI, Pb, Bi. Bronsted acid catalysts include, but are not limited to, HCl, H2SO4, HNO3, H3PO4, HClO4. Preferably the catalyst is AlCl3, NH4Cl, or a mixture thereof. For example, a 28% aqueous solution of AlCl3 can be used. Other examples of hardener compositions are given in the examples below. One of ordinary skill in the art will be able to readily identify hardeners that could be used with the MUF resin of the invention.
The hardener includes a thickener or a thickening material. The thickening material can be anything that increases the viscosity of the hardener. The viscosity must be high enough to avoid separation of the suspended material, i.e., the filler, for at least two weeks, preferably at least four weeks. Typically, the thickener increases the viscosity of the hardener to greater than about 3000 cps, preferably greater than about 10,000 cps, and more preferably greater than about 20,000 cps. Any thickener known to those of ordinary skill in the art can be used. Examples of thickening materials include, but are not limited to, SER-AD FX 1100 (commercially available from Condea Servo LLC of New Jersey), a polyurethane thickener; a polyvinyl alcohol; a fumed silica; a bentone clay; and mixtures thereof. Preferably, the thickening agent is a polyurethane thickener. Optionally, the thickening agent may also be added to the resin. Typically, when a polyurethane thickener is used, the thickener will be about 0.8% to 2.5% of the hardener. When a polyvinyl alcohol or acetate thickener is used, the thicker will be about 8% to 10% of the hardener.
The hardener also includes one or more fillers that are insoluble in water. The fillers help to fill the gaps between wood components when they are bonded together. It is very important that the filler material be present to sufficiently fill in the gaps. The filler material is suspended in the liquid portion of the hardener. Any filler known to those of ordinary skill in the art could be used, such as organic fillers including, but not limited to, wood flour, pecan shells, and other cellulosic materials including, but not limited to, organically modified cellulose, various starches, and insoluble fibrous carbohydrate fillers. Inorganic fillers that may be used include, but are not limited to, kaolin, tricalcium phosphate, clays, and barium sulfate. Other inorganic fillers include hydrated aluminum magnesium silicates, and others known in the art. Preferably, the inorganic filler is Attagel 40, a hydrated aluminum magnesium silicate (commercially available from Engelhard Co. of New Jersey). In one embodiment the wood flour may be present in an amount of about 7% to 10% by weight, and the pecan shells are present in an amount of about 2% to 8% by weight of the hardener. The resulting hardeners are stable and do not separate or precipitate, for at least two weeks, preferably for at least four weeks.
The hardener may also optionally contain anti-foaming agents. An anti-foaming agent prevents excess foaming and frothing when the ingredients of the hardener are mixed together. Examples of anti-foaming agents include, but are not limited to, silicones fluorocarbons, and other surface active agents.
Commercially available hardeners can also be used with the resin, such as for example, Cycat(copyright) HF475, an NH4Cl catalyzed powder type hardener (commercially available from Cytec Industries, Inc. of Stamford Conn.), or other similar commercially available hardeners.
The liquid MUF resin prepared by the process of the invention can be combined with a liquid hardener to form the adhesive. Any hardener known to those of ordinary skill in the art can be used. Preferably, the hardener is the hardener of the invention. The resins will cure when mixed with the hardener in wood bonding applications. The adhesive can be used to join two or more pieces of wood together and when the adhesive cures the pieces of wood become bonded together. The resins of the present invention generally cure in about three hours with 10% hardener at room temperature. The curing time is reduced with the addition of more hardener. For wood bonding applications, the higher formaldehyde to melamine ratio in the MUF resin of the present invention advantageously increases the cure speed for a given level of hardener, and, thus, reduces the necessary curing time. Typically, the adhesive contains about 5 to 35 parts per hundred (pph) of hardener, with the balance being the resin. Preferably, the adhesive contains about 10 to 20 pph of hardener.
Rather than a room temperature cure, two methods of curing are generally used. The first is radio frequency curing. A laboratory size 4 KW SL3648 Slant Adhesiver (commercially available from Radio Frequency Services, Inc. of Wilkesboro, N.C.) may be used for this type of curing. The curing time is generally about 90 seconds or less for the compositions of the present invention. The ionic compounds used as catalysts for the hardener make the adhesive conductive. The conductivity of the adhesive provides for heat generation under radio frequency conditions that leads to curing. The hardeners have the largest influence on the conductivity of the adhesives. A hardener with an AlCl3 catalyst will generally have a lower conductivity, while a hardener with a NH4Cl catalyst will generally have a higher conductivity, when the same amounts of catalyst are used. A lower conductivity generates less heat under radio frequency conditions. If the temperature of the adhesive line is too low, the adhesive cure may be incomplete, thereby generating a weaker bond. Typically, the conductivity of the adhesive is about 1 to 5 mS/cm, preferably about 2 to 4 mS/cm.
The conductivity may be measured using any method known to those of ordinary skill in the art. For the examples below, the conductivity was measured using a 53 Electrodeless Conductivity Analyzer (commercially available from GLI International of Milwaukee, Wis.). The adhesive is placed in a beaker or similar container, and the probe of the analyzer is submersed therein. The analyzer measures the disturbance of the electric flux emitted by the probe.
A second method of curing is thermal press curing. A Carver Lab Press (commercially available from Fred S. Carver, Inc. of Summit, N.J.) may be used for this type of curing. The composition is pressed for about 4 to 5 minutes at a pressure of about 5,000 pounds and a temperature of at least about 120xc2x0 C., preferably at least about 148xc2x0 C.
These adhesives perform satisfactorily in wood bonding applications. Surprisingly, the liquid MUF resin-liquid hardener system of the present invention has excellent adhesive properties. As shown in the examples below, the wet shear performance is especially good. Liquid systems generally do not have good adhesive properties. The adhesive line, i.e., the line connecting two pieces of adhesived wood, is stronger compared to prior art adhesives, especially when it is exposed to wet environments than other adhesives.
Another advantage of this invention is that the resin and the hardener are generally both in liquid form. The prior art has generally used a powdered resin and powdered hardener. Powder materials, however, have the disadvantage of causing dust that is unwanted and can be environmentally harmful. Thus, having both the resin and hardener in liquid form is more convenient, more hygienic, and reduces labor costs. The labor costs are reduced because less mixing is required and the cure time is reduced.
The resins of the invention also has a shorter cure time than conventional resins and harden faster than prior art resins when the same hardeners are used.